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Rb-Sr and Sm-Nd Dating 9/7/10

Rb-Sr and Sm-Nd Dating 9/7/10. What are the principles behind Rb-Sr and Sm-Nd dating? What processes can these dating systems address? What are the main limitations of these methods?. Lecture outline: dating principles & techniques Beyond dating - tracking igneous processes

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Rb-Sr and Sm-Nd Dating 9/7/10

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  1. Rb-Sr and Sm-Nd Dating 9/7/10 What are the principles behind Rb-Sr and Sm-Nd dating? What processes can these dating systems address? What are the main limitations of these methods? • Lecture outline: • dating principles & techniques • Beyond dating - tracking igneous • processes • The seawater Sr, Nd isotopic curves Photo of Fe-Ni (left) and chondritic (right) meteorites

  2. 87Rb-87Sr decay scheme -decays to 87Sr by β-, half-life=48.8 billion years Rb/Sr ratios for various rocks: Ultrabasic 0.2 Basaltic 0.06 Granites 0.25-1.7 Shale 0.46 Sandstone 3 87Rb=27.83% 85Rb=72.17% 88Sr=82.53% 87Sr=7.04% 86Sr=9.87% 84Sr=0.56% ALL STABLE What accounts for huge range in Rb/Sr ratios of rocks? 1. Rb subsitutes for K in K-bearing minerals while Sr substitutes for Ca in Ca-bearing minerals 2. Rb and Sr are fractionated by igneous processes: Rb tends to prefer melt (more “incompatible” than Sr) Bottom line: High Rb/Sr rocks contain more 87Sr Low Rb/Sr rocks contain less 87Sr

  3. Igneous Processes and 87Sr/86Sr ratios MORB * Remember that 87Rb likes melt 87Sr/86Sr ratios of igneous rocks: MORB 0.7025 Continents 0.7119 Ocean Islands >0.704 vs. Meteorites 0.699

  4. 87Rb-87Sr decay equation Rb/Sr= 0.8 Rb/Sr=1.2 ROCK (87Sr/86Sr) i= 0.702 Rb/Sr=0.6 measured measured when you crystallize a rock, you will always have some Sr present t=Time of crystallization So how do you determine the initial 87Sr/86Sr ratio? Because igneous rocks are so heterogeneous, different mineral phases will have different Rb/Sr ratios, even though they have the same crystallization age and the same 87Sr/86Sr initial. MANTLE 87Sr/86Sr = 0.702

  5. 87Rb-87Sr isochrons A schematic Rb-Sr isochron measured measured when you crystallize a rock, you will always have some Sr present Sample with lower [Rb] Sample with higher [Rb] Bushveld granite Rb-Sr isochron If x=(87Rb/86Sr)m And y=(87Sr/86Sr)m We have y=b+mx Where intercept b=(87Sr/86Sr)i And slope m=(eλt-1)

  6. More than just an age tool - tracking (87Sr/86Sr)i through time BABI - Basaltic Achondrite Best Initial = Bulk Earth, undifferentiated Rb-Sr isochron From meteorites 87Sr/86Sr ratios of igneous rocks: MORB 0.7025 Continents 0.7119 Ocean Islands >0.704 vs. Meteorites 0.699 T=4.5Ga Questions: 1. Why are all present-day (87Sr/86Sr) values greater than BABI? 2. Why are continental values the highest?

  7. More than just an age tool - tracking (87Sr/86Sr)i through time Average continental crust continuing continental growth enriched early continental differentiation BABI Ocean islands MORB depleted continuing upper mantle depletion A rock’s (87Sr/86Sr)i value call tell you how enriched or depleted its mantle source was. i.e. (87Sr/86Sr)i = 0.7020 at 2Ga means a depleted source How would you explain a (87Sr/86Sr)i value of 0.728 at 1.4Ga?

  8. Rock-forming complexities and (87Sr/86Sr)i Ex: Mt. Shasta lavas span a wide range of Sr isotopic chemistries As crystals form, Rb enriched in melt, eventually can get ultra-enriched (87Sr/86Sr) Crystals form in magma chamber, Rb stays in melt Or magma melts host rock, which has high 87Sr/86Sr Or magma chambers with different histories mix prior to eruption

  9. Seawater (87Sr/86Sr) through time Controls on Seawater Sr Isotopic composition Questions: Why is the river Sr isotope value the highest? Why is the hydrothermal Sr isotope value the lowest? Why is carbonate recrystallization Sr isotope value equal to that of seawater? Sr flux rate Sr isotope ratio Seawater Sr Isotopic Curve (as measured on old and young carbonates) mountain- building hydrothermal activity Himalayan uplift

  10. Introduction to Rare Earth Elements - REE so named because we could not measure them until high-precision mass spec techniques developed - all REE have 3+ charge, ionic radii decrease with increasing Z - all REE are “incompatible” (they prefer the melt), but light REE are more incompatible (Nd prefers melt more than Sm)

  11. 147Sm-143Nd decay scheme -decays to 143Nd by α, half-life=106 billion years Sm/Nd ratios for terrestrial materials: garnet 0.539 MORB 0.32 seawater 0.211 Shale 0.209 Solar 0.31 147Sm=15% 4 other isotopes 143Nd=12.2% 6 other isotopes Nd has a lower ionic potential (charge/radius) than Sm, so the bonds it forms are weaker. Nd is concentrated in melt, while Sm remains in solid. So… High Sm/Nd rocks contain more 143Nd Low Sm/Nd rocks contain less 143Nd NOTE: Sm parent will be enriched in “depleted” sources (i.e. MORB) (opposite to Rb/Sr system, where parent enriched in continents)

  12. 147Sm-143Nd isochrons virtually same equation as for Rb/Sr system • - measured by isotope dilution • and mass spectrometry If x=(147Sm/144Nd)m And y=(143Nd/144Nd)m We have y=b+mx Where intercept b=(143Nd/144Nd)i And slope m=(eλt-1)

  13. Epsilon Nd notation • CHUR = “Chonritic Uniform Reservoir” • - typically measured on chondritic meteorites (DePaulo and Wasserburg, 1976a) • - If CHUR and samples are measured in the same lab, then regardless of normalization and corrections, one can compare εNd values • also practical way to report very small Nd isotope changes (~0.0001) • So what’s the ε value for CHUR?

  14. CHUR model ages - Nd isotope “model ages” can be calculated which represent the time of separation from CHUR evolution - can also calculate an age from assuming a “depleted mantle” evolution (substitute DM Nd isotope values into equation below) Present-day ratios: 147Sm/144Nd 143Nd/144Nd CHUR 0.1967 0.512638 DM 0.222 0.713114 “Model Age” calculation:

  15. Sm-Nd and Rb-Sr cross-plots

  16. Seawater Nd isotopic evolution - Nd is not well-mixed in the ocean, because it has a short residence time Rt= total reservoir / (Σsinks) - anything with a residence time shorter than the turnover time of the ocean (~1500y) will exhibit concentration and isotopic variability in seawater - REE are extremely resistant to metamorphism, so can measure Nd isotopes in very old sediments, fish teeth, ferromanganese nodules, etc - Nd isotope variations reflect large-scale tectonic history Ferromanganese nodules on Pacific Ocean floor

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